Petroleum, coal, and natural gas are three indispensable resources that support modern civilization and have propelled the rise of advanced technologies. Presently, fossil fuel resources account for 86% of the world's energy supply, as well as 96% of its organic chemicals (US National Petroleum Council, 2007). These values are unsustainable, given the increasing demand for energy and chemicals coupled with the diminishing supply of fossil fuels. In addition, rising CO2 emissions, decreasing accessibility to fossil fuel resources, and increasing consumer costs mandate a reduction in fossil fuel consumption (Kramer & Haigh, 2009; Tilman et al. 2009). Abundant, renewable biomass resources are able to meet this rising demand and alleviate these environmental and socioeconomic concerns.
If biomass is to replace fossil fuels as an energy and chemical resource, it must match the wide array of products derived from fossil fuel reserves. Obtaining furans from biomass is of significant current interest. The six-carbon furanic 5-(hydroxymethyl)furfural (HMF) holds great potential to meet this challenge (Antal et al., 1990; Chheda et al., 2007; Huber et al., 2005); Lewkowski, 2001; Roman-Leshkov et al., 2007; Roman-Leshkov et al., 2006). The carbon skeleton of HMF is identical to those found in the hexose sugars, which are the primary components of cellulose and hemicelluloses found in biomass. Additionally, by utilizing straightforward chemical methods, HMF can be transformed into a variety of useful products including acids, aldehydes, alcohols, and amines, such as common polyester building blocks 2,5-furandicarboxylic acid, 2,5-bis(hydroxymethyl)furan, and 2,5-bis(hydroxymethyl) tetrahydrofuran (Chheda et al., 2007; Huber et al., 2005; Lewkowski 2001; Roman-Leshkov, 2006) as well as the promising liquid fuel 2,5-dimethylfuran (Roman-Leshkov, 2007).
Furfural is perhaps the most common industrial chemical derived from lignocellulosic biomass with an annual production of more than 200,000 t (Kamm et al. 2006). The conversion of pentoses into furfural has been reported (Sproull et al., 1985; Moreau et al., 1988; Mansilla et al. 1998; Dias et al. 2007). Most industrial processes achieve yields in the range of 50 molar %, which may be limited by homopolymerization and condensation with unreacted xylose. In typical processes reported, Brønsted acidic catalysts were used in aqueous solution at temperatures greater than 150° C. (Moreau et al., 1998; Dias et al., 2007).
The conversion of cellulose to HMF proceeds through three steps: hydrolysis of cellulose to glucose, isomerization of glucose to fructose, and dehydration of fructose to HMF (see Scheme 1, FIG. 1, below). A number of processes have been reported to transform glucose and fructose into HMF, though few processes can access HMF in high yields directly from cellulose (Moreau et al., 2000; Seri et al., 2001; Watanabe et al., 2005a; Watanabe et al., 2005b; Yan et al., 2009; Tyrlik et al., 1999; Hu et al., 2009; Sthlberg et al. 2010; Zhao et al. 2007; Pidko et al., 2010). Both xylan and xylose can be dehydrated into furfural (Zeitsch, 2000; Mamman et al. 2008). Scheme 2 illustrates the formation of furfural from xylan and xylose. In this scheme, the C-2 hydroxyl group is displaced to form a xylose-2,5-anhydride and subsequent dehydration steps produce furfural (Antal et al., 1991; Nimlos et al., 2006).
An important goal in the art is direct conversion of biomass to HMF and other furans. Recent developments in conversion technologies using solid acid or base (Carlini et al., 1999; Zhao et al. 2011), or heavy metal catalysts (Binder & Raines, 2009; Su et al., 2009) have shown progress toward this goal. The lack of cellulose conversion by solid acid catalysts and the reported environmental toxicity of heavy metals represent practical problems for implementation of such processes. With today's emphasis on “green” chemistry, it is very desirable that a conversion process to HMF use mild reaction conditions to transform cellulose as well as other carbohydrates with recyclable and environmentally benign reagents, catalysts, and solvents. While processes for generating HMF, furfural and other furans are known in the art (e.g., U.S. Pat. Nos. 7,572,925 and 7,880,049; US 2008/0033187; Zhao et al. 2007; JP 2005232116) there remains a need in the art for conversion processes that efficiently and selectively convert carbohydrates to HMF and other furans. Such efficient and selective processes must be available for biomass to become a viable feedstock for energy and chemicals.
Boric acid is an environmentally benign catalyst that is utilized for the dehydration of alcohols (Brandenberg & Galat, 1950; O'Connor & Nace, 1955).
However, boric acid and phenylboronic acid have both been reported to be inhibitory towards the hydrolysis of cellulose and the dehydration of sugar monomers in a non-aqueous medium; and inhibition was indicated to be due to the stability of cyclic boronate esters to decomposition and dehydration (Kawamoto et al. 2008a; Kawamoto et al. 2008b). More specifically, Kawamoto et al. 2008a reports that boric acid inhibited acid-catalyzed depolymerization of cellulose, and inhibited the formation of dehydration products, including HMF, in sulfolane at high temperature. Kawamoto et al. 2008b reports that boric acid and phenylboronic acid suppressed acid-catalyzed dehydration and formation of furfurals from levoglucosan. In both reports, inhibition or suppression is indicated to be due to stable boronate complex formation. However, Hansen et al. 2011 and Sthlberg et al. 2011 recently reported that boric acid itself can promote dehydration of hexoses to HMF.